Such process for producing synthetic fuels (so-called MtSynfuels process; MtSynfuels=Methanol to synthetic fuels) is known from WO 2006/076942 A1. In an olefin reactor, a mixture consisting of oxygenates, such as methanol and/or DME, and steam initially is reacted on a zeolite catalyst to obtain short-chain olefins. As by-products, paraffins and aromatics are obtained. Subsequent to the olefin production, the olefin mixture obtained in the olefin reactor is densified, partly condensed, and in a second process stage the condensate is oligomerized to long-chain hydrocarbons. In the process, the aromatics are alkylated. In a succeeding separating means, the oligomerized product stream is divided into a product stream containing synthetic fuels (LPG, gasoline, heating gas and, upon hydrogenation, Diesel/Kerojet) and streams containing unsaturated and saturated hydrocarbons, respectively. The saturated hydrocarbons are recirculated to the olefin reactor, whereas the unsaturated hydrocarbons are recycled to the oligomerization reactor.
In the conventional MtSynfuels process, the olefin reaction usually is performed at a low pressure of 1 to 2 bar. This requires large apparatuses and machines and therefore leads to high investment costs.
The alkylation of the aromatics in the oligomerization reactor leads to a reduction of the cetane number achievable in the Diesel, as the cycloalkanes obtained therefrom in the succeeding hydrogenation have low cetane numbers. A cetane number which is too low can lead to an ignition delay between the injection and the self-ignition of the fuel, and hence to an abrupt, explosion-like combustion of fuel with a loud combustion noise.